The present invention relates generally to industrial control systems. In particular, the invention pertains to control methods for investment casting shell handling systems, and methods of autonomous control of groups of machines through a programmable logic controller.
Investment casting systems have recently increased in complexity to fabricate more intricate and complex metal parts. In the past, producing an investment casting required the relatively simple steps of surrounding a wax or foam mold with sand in a gondola into which the molten metal is poured in a sacrificial or molten replacement of the mold. Automation consisted of moving a gondola mounted on a railed conveyor to a particular station at which a particular processing step of the casting within the gondola carrier was completed. Mainly, these steps consisted of pouring sand into the gondola to surround the sacrificial mold and then pouring a metal alloy into the mold. Control systems consisted of operator initiated switching and preset loading operations based upon gondola positioning. Intelligent control was a human operator initiating each machine in sequence based upon known timing constraints.
However, investment casting steps have evolved to be able to produce far more complex parts using a variety of alloys. Today, casting xe2x80x9cshellsxe2x80x9d made up of successive layers of ceramic materials are built up around a sacrificial wax or foam mold. These shells fully encase the mold and functionally replace the bulky gondola and sand supports previously used. Once a suitable shell is created around the mold, molten metal is poured directly into the shell through a shaped cemented into the shell during its fabrication. The sacrificial core is molten or vaporized and the shell is extracted from the newly cast part during the clean-up process.
Building up the successive layers of ceramic material around the mold requires a sequence of repetitive steps of dipping the molds into various mixtures of glue/cement slurries and then surrounding the coated mold with fluidized sand and drying. The duration and environmental conditions of each drying step, in conjunction with the type of sand applied to the specialized slurry coating greatly affects the properties of the final shell created. Therefore, specific xe2x80x9crecipesxe2x80x9d are designed for each particular casting shell part to achieve each shell""s desired properties.
Due to the many variations within recipes, automation of shell manufacturing is complex. Robot manipulators, fluidized barrels or rainfall sanders, temperature controlled ventilation fans, and conveyors carrying wax or foam molds must work in a coordinated effort to make a desired casting shell in accordance with a specified recipe. Furthermore, different types of shells for casting different types of metal parts are often made on the same shell assembly line, utilizing the same machines. In order to automate manufacturing of the various types of shells on one assembly line, xe2x80x9ccellsxe2x80x9d of processing machines must be able to automatically recognize what type of shell has entered the production line and automatically configure their processing steps in accordance with a particular recipe associated with the shell part. Typically, this will entail automatic recognition of the shell part through radio frequency or bar coded tags affixed to the part. Also, multiple conveyors must move in a coordinated effort, sometimes throughout a large facility, to and away from each processing cell.
Previously, during the evolutionary advancement of industrial controls, robot manipulators and other processing machines included relatively simple programmable memory which was preprogrammed to initiate tasks in response to external conveyor sensors. Little or no communication occurred between each machine and overall system level control rudimentary. These machines were therefore mostly autonomous and acted as a master with respect to any connected programmable logic controllers (PLCs). The PLCs were simply programming conduits through which individual robots could be programmed.
In response to the necessity to coordinate robot and machine actions, newer systems have included real-time databases on the factory floor to which machines are connected through PLCs. In these types of xe2x80x9creal-timexe2x80x9d systems, a processor, typically the CPU in a Personal Computer located on the factory floor, accesses data elements in a resident database and in response issues commands to the machines through the PLCs. In these newer arrangements, machines and manipulators receive their movement instructions through the PLCs, which act as a bi-directional pass-through multiplexer to which multiple robot and machines might be connected.
However, high speed complex processing on a factory floor tends to be less reliable than remote processing away from resident electromagnetic pulse (EMP) interference. Furthermore, factory floor data communications necessitates multiple error correcting protocols and hinders the speed at which data may be transmitted. For example, Allen Bradley""s well known Data Highway Plus(trademark) network transmits data at 240 k bits/sec for networks extending to 10,000 feet. This is slow in contrast to the nominal local area networks which transmit data a 10 to 100 Mbits/sec rate. Currently, with the addition of proper shielding, such networks are beginning to be installed directly on the factory floors allowing increased data communications rates between PLCs. However, older, more reliable networks, such as the DH+ are the norm, and the added shielding expense and increased error rates are prohibitive.
For complex investment casting shell handling operations in which dozens of machines, sensors, and conveyors, in dozens of different processing cells must communicate, factory floor networks limit the processing power of the CPU accessing the real-time data from the floor network. Moreover, complex processing in an application server or PC results in limiting factory floor operations by making the distributed PLCs dependent upon real-time commands from a control application running on the server.
From the foregoing, modem casting shell systems require an information topology and method in which complex processing can be removed from the factory floor and commands automatically distributed to PLCs on the factory floor in a predictive manner, and from which sensor and status information can be retrieved and displayed at remote locations. In effect, a need exists for a factory processing system in which distributed PLCs on the factory floor become individual master controllers over machines in associated individual processing cells, and from which a remote industrial control application server may service a plurality of individual master PLCs on a factory floor at the request of an individual PLC.
It is the object of the present invention to provide a predictive industrial control system in which a computer server can access and process information from local and remote databases and then pass groups of commands to programmable logic controllers on a factory floor upon request.
Another object of the present invention is to provide an autonomous control topology in which groups of commands may be downloaded into programmable logic controllers on the factory floor for controlling connected machines.
A further object of the invention is to provide an industrial control system in which programmable logic controllers on the factory floor act as master controllers for cells of connected machinery.
A still further object of the present invention is to provide a unique control word format or encoding casting shell processing commands.
Another object of the present invention is to provide processing scalability of multiple processing cells in response to increased production activity.
And yet another object of the present invention is to provide distributed processing on an industrial production line through multiple programmable logic controllers.
In summary, the invention is an industrial control system for controlling individual cells of machines through multiple PLCs in an investment casting processing and handling operation. An application server located in a remote, non-industrial environment executes an industrial control application (ICA) utilizing tagged reference names corresponding to individual machines and information input devices such as sensors on the production line. The ICA accesses local databases including a Dynamic Data Exchange (DDE) database for holding PLC control status information and a Structured Query Language (SQL) database for holding casting shell part numbers and their associated recipe control words. The ICA monitors the DDE database and as a new shell part number appears on a load conveyor in the production line, Radio Frequency (RF) tags on the part update the DDE database and signal the ICA to transfer the appropriate recipe control words and robot moves required to process the part to the primary processing cell""s PLC. The PLC then controls each machine in accordance with the downloaded commands from the ICA. The control program downloaded into the PLC at boot-up is written such that machines connected to the PLC may operate independently from the ICA during a selected number of dip cycles. Since processing instructions are downloaded in a predictive manner well ahead of any situation requiring new instructions, the processing of the ICA instructions on the remote server or communications on a relatively slow multiple error correcting protocol network such as DH+ does not limit the processing throughput each cell; and therefore, the production line throughput. Furthermore, the system allows for alteration of the sequence of processing steps from automatic to manual modes at will without seriously hampering the system throughput.
Other features and objects and advantages of the present system will become apparent from a reading of the following description as well as a study of the appended drawings.